(1) Technical Field
This invention generally relates to electronic circuitry, and more specifically to switching devices particularly suited for use with radio frequency (RF) field effect transistors (FETs).
(2) Background
Electronic circuitry often uses FETs as electrical switches, resistors, and/or capacitors. One such usage of FETs is in integrated circuit RF switches. An RF switch is a device to route RF signals through transmission paths, such as between an antenna and multiple transceivers in a radio system; an example of such a radio system is a cellular telephone.
FIG. 1 is a schematic diagram of a typical prior art RF single-pole, multiple-throw switch circuit 100. Radio frequency signals can be selectively coupled between a common connection port RFC and at least two selectable RF ports (four such ports, RF1-RF4, are shown). The common connection port RFC is typically connected to an antenna (either directly or through additional circuitry, such as RF filters). The selectable RF ports RF1-RF4 are typically used to connect the RFC port to other circuitry (not shown), such as RF transceivers (again, either directly or through additional circuitry).
In operation, if selectable port RF1 is selected to be coupled to the RFC port, then an associated series switch 1021 is closed (i.e., switched “ON”) to complete the port coupling, and an associated shunt switch 1041 coupled between the selected port and circuit ground is opened (i.e., switched “OFF”). Concurrently, the associated series switches 1022-1024 for the other selectable ports RF2-RF4 are opened and their associated shunt switches 1042-1044 are closed, thereby shunting each of the open ports to ground. The various series and shunt switches are opened or closed in similar fashion to couple any other selectable port RF2-RF4 to the RFC port. All of the switches are typically implemented as FETs on an integrated circuit (IC) die or “chip”. Not shown is the conventional control circuitry for selecting and unselecting ports.
The closed shunt switches result in improved isolation of the switch 100 by shunting the open ports RF2-RF4 to ground; such a configuration results in short reflective unselected ports. However, a short reflective port RF switch 100 of the type shown in FIG. 1 is not suitable for all applications. In particular, such an architecture will short out the load impedance connected to an unselected port. The present invention addresses such shortcomings.